In this paper, we present evidence that geomagnetic activity has the potential to disrupt the electricity flows between Ontario, Canada, and New York State, USA. This conclusion is based on a modeling framework that makes use of weather data, electricity load data, measures of transmission "network effects," proxies for geomagnetically induced currents and the expected level of power grid conditions. The model is estimated using hourly data throughout 1 May 2002 through 31 October 2003. The model is evaluated using out-of-sample hourly data over the period of 1 November to 9 December 2003. The out-of-sample predictions are more accurate when the forecasting equation reflects the estimated contribution of geomagnetic activity. The structural modeling results for the sample period indicate that the peak predicted effect of geomagnetic activity on the electricity flow was about 1,604 MWh in absolute value when the geomagnetically induced current proxy achieved a value of 363.2 nT/min. The analysis also indicates that the level of the geomagnetically induced electricity flow is highly dependent on ambient temperature and expected system conditions at the time of the geomagnetic storm. Moreover, geomagnetic activity is an important driver of the volatility in the electricity flows. The overall findings indicate that the scope of the challenge posed by space weather to the operations of the electric power system is likely understated.